The invention concerns a mold for so-called flat treads of tire casings, that is, intended to be molded flat, notably, for the retreading of tire casings.
The invention concerns, in particular, a mold containing first and second molding parts respectively defining the outer surface and inner surface of the tread, at least one of the parts being movable between two positions corresponding to the opening and closing of the mold.
Patent EP-A 0,403,443 describes a tread mold, comprising first and second molding parts respectively defining the outer and inner surfaces of the tread, those molding parts delimiting the tread molding space.
In order to ensure sufficient and uniform pressure on the tread, it is necessary to adjust the molding space to the volume of said tread. But the molding phase generally begins before complete closing of the mold, which entails risks of flow of the rubber out of the tread between the two molding parts. Those zones of rubber xe2x80x9ctrappedxe2x80x9d between the molding parts constitute, at mold outlet, defects commonly known as xe2x80x9cflashes,xe2x80x9d which need to be removed manually.
Patent JP No. 64-77390 presents a flat tread mold making it possible to facilitate the operation of elimination of flashes on the tread. In that mold, cavities receiving a possible overflow of rubber are juxtaposed outside the lateral edges of the molding part defining the outer surface of the tread. The lateral edges further present beveled faces, so that the flashes originating from molding present a marked groove delimiting the desired contour of the tread.
The construction of such a mold therefore simplifies the flash elimination operation, but does not deal with the source of the problem, which consists of preventing the formation of flashes. In fact, even with a facilitated flash elimination operation, flashes constitute a loss of time and material.
Furthermore, in a totally rigid mold the volume expansion of the tread at the time of vulcanization can produce a considerable increase of pressure and appreciable wear on some parts of the mold.
One solution is by control of pressure making it possible, when a predetermined threshold has been reached, to let the two molding parts separate from each other very slightly, for example by a reduction of the external pressure exerted on the movable molding part, and thus to extend the molding space in a direction parallel to a transverse plane of the tread. However, on such expansion, rubber coming out of the tread can flow to the interface of the two molding parts, again causing the appearance of flashes.
The invention is aimed at overcoming all of the foregoing difficulties.
The position of complete closing of the mold will be described below as xe2x80x9cclosed position of the moldxe2x80x9d and a position of the mold slightly separated from the position of closing of the mold in a direction parallel to a transverse plane of the tread will be described as xe2x80x9cslightly-open position of the mold,xe2x80x9d the expression xe2x80x9cslightly separatedxe2x80x9d corresponding to a mold opening in the order of a few millimeters.
According to the invention, the mold for flat treads of tire casings comprises first and second molding parts respectively defining the outer surface and the inner surface of a tread, at least one of the molding parts being movable between two positions corresponding to opening and closing of the mold, and is such that one of the molding parts bears at least one elastically deformable element securing molding of the longitudinal edges of the tread surface concerned and capable of forming and maintaining a tight connection of the two molding parts between the closed position and a slightly-open position of the mold.
Thus, that mold makes it possible to obtain treads without flashes between the molding parts, whatever the origin of the latter: arising out of molding on closing of the mold or out of vulcanization. Of course, for optimum operation of the mold, it is important to have great precision, such as a tolerance of approximately 1.5% of the volume of rubber entering the mold.
The invention therefore concerns a mold forming a tight connection of the molding parts due to the elastic deformation of a molding element upon closing of the mold, that is, from the start of molding and, therefore, before complete closing of the mold and when a slight separation of one of the molding parts is undertaken at the time of vulcanization.
Other characteristics and advantages of the invention will appear on reading of a description of working example of a mold according to the invention with references to the drawings.